1. Field of the Invention
The present invention relates to an image stabilization apparatus that compensate image blur (image degradation) caused by shakes such as hand shakes, and relates to a control method for such an image stabilization apparatus, an optical apparatus, and an imaging apparatus.
2. Description of the Related Art
At present, cameras provided with image stabilization apparatuses that prevent image blur caused by hand shakes and are configured with, for example, a shake compensation unit, a driving unit, and a shake detection unit are in commercial production, and as a result, user-caused shooting mistakes are decreasing.
Camera shake caused by hand shakes of a user is usually within a frequency range of 1 to 10 Hz. In order to capture a blur-free photo even if such hand shakes occurs at the time of shutter release, a unit is conventionally provided that detects camera vibrations caused by hand shake and moves a lens for correcting image blur (hereinafter referred to as a “correction lens”) according to the detected values.
In order to capture a blur-free photo even if camera vibrations occur, it is necessary to accurately detect camera vibrations and correct shifts of an optical axis caused by vibrations such as camera shakes. In principle, detection of camera vibrations (camera shakes) is possible by incorporating, in a camera, a vibration detection unit that detects acceleration, angular acceleration, angular velocity, angular displacement or the like and processes the output as appropriate. Then, by an image stabilization unit (specifically, correction lens) being driven by a driving unit, the image stabilization unit whose optical axis is moved based on the vibrations from the vibration detection unit, image blur control is performed.
As an example, a stabilization control apparatus that detects rotational shakes by using an angular velocity sensor and moves a part of the lens or an image sensor so as to reduce vibrations on the image sensor surface is incorporated in various optical apparatuses as an effective vibration correction function.
However, when shooting at close ranges, shooting at a high magnification ratio, and so on, image degradation caused by what are known as “translational shakes” (parallel shakes), which act in the translational or vertical direction relative to the optical axis of the camera and cannot be detected solely by an angular velocity sensor, cannot be ignored. It is necessary to actively detect and compensate translational shakes in, for example, the case where an image is shot from approximately 20 cm from the subject, as in macro shooting, the case where the focal length of the photographing optical system is extremely long (for example, 400 mm) when the camera is approximately 1 m away from the subject, and so on.
Japanese Patent Laid-Open No. 7-225405 discloses a technique where an acceleration sensor that detects acceleration is provided, translational shakes are found from a second-order integral of the acceleration obtained by the acceleration sensor, and a shake compensation unit is driven based on the translational shake and the output of a separate angular velocity sensor.
However, when shooting a moving image, there are cases where it is better to weaken translational shake correction depending on the in-focus area of the main object. An example of such a case is close range shooting. During close range shooting, because a significant error occurs in the correction amount due to the difference in magnification ratio, if the vibration correction amount is not adjusted for each object distance, it is not possible to perform good vibration correction on the entire screen. Specifically, in the case where sufficient vibration correction is performed on a main object, the distance to which is 10 cm, its background (for example, the object distance is 1 m) is not sufficiently corrected, instead, the image blur may become worse (overcorrection). Particularly when shooting a moving image, unlike still image shooting, its influence is constantly recorded, which may cause user discomfort.
This will now be described in further detail. As described above, there are two types of vibrations affecting an imaging apparatus: rotational shakes in which the imaging apparatus swings about the center of rotation; and translational shakes in which the entire imaging apparatus moves in parallel. Image degradation due to rotational shakes becomes worse as the object distance and the focal length of the imaging apparatus become longer. Image degradation due to translational shakes is greatly correlated with the object distance and the focal length (image magnification), and thus the degradation becomes worse as the image magnification becomes larger (as the object distance becomes shorter and the focal length becomes longer). Under ordinary photographic conditions (for example, when the object distance is 1 m), the influence of image degradation by translational shakes can be substantially ignored.
However, in the case of close range shooting (for example, when the object distance is 10 cm), because the magnification ratio is high, the influence of image degradation by translational shakes cannot be ignored. In this case, for example, an optical image stabilization system performs detection using an acceleration sensor or the like, and an electronic image stabilization system detects translational shakes by detecting image blur, and corrects the vibrations according to the results of detection.
However, various objects at different distances are present in the screen. Accordingly, when vibration correction is performed with respect to the distance to the main object, the main object can be prevented from image degradation due to translational shakes. On the other hand, with respect to the background and the other objects at different distances, the vibration correction is not sufficiently performed, instead, it may cause image degradation.